The present invention relates to a method for shaping a pipe from a tetrafluoroethylene resin (hereinafter referred to as "TFE resin"), and more particularly to a shaping method of a TFE resin pipe wherein a TFE resin porous sheet which is not sintered is wound around a mandrel such as a bar or tube and it is sintered.
There are various methods for shaping TFE resin pipes, among which the important ones are as follows:
(I) a method wherein a mold is charged with a TFE resin powder and pressed to obtain a preform, and then the preform is sintered. PA0 (II) a method wherein a TFE resin is extruded in a gel state with heating (so-called "ram extrusion") into a pipe. PA0 (III) a method wherein an unsintered TFE resin sheet is wound around a mandrel and then sintered.
Among the shaping methods of TFE resin pipe, the winding method (III) is superior to other methods, because, according to the winding method (III), it is possible to obtain a pipe having a desired length or a desired thickness by controlling an amount of the TFE resin sheet to be wound, to obtain a pipe having a desired inner diameter according to an external shape of a mandrel, to easily obtain laminated pipes, and to obtain a circular and thin pipe.
The method (III) is generally conducted as follows: An unsintered TFE resin sheet is spirally wound around a mandrel having an external shape corresponding to an internal shape of a pipe to be obtained so as to overlap a turn with a next turn until the desired length and wall thickness are attained,. and it is sintered with heating. Also, reinforced pipes can be easily shaped by winding materials different from the TFE resin such as wire net, glass cloth, steel wire together with the TFE resin sheet to set the different sheet layer between the TFE resin sheet layers or on the TFE resin sheet layer, and further, a pipe provided with a flange can be shaped. (see, for instance, Japanese Examined Patent Publication No. 59-25647).
As mentioned above, though the winding method has various advantages, it also has a disadvantage such that many voids appear on sintering, thus the bulk density of the disadvantage, such a method for improvement that glass cloth is wound around the outside of the wound TFE resin sheet before sintering has been proposed, but this method has no effect when the thickness of the wound TFE resin sheet is thin, and moreover bears such a drawback that the surface of the sintered pipe turns rough.
An object of the present invention is to provide a method for shaping a TFE resin pipe which is dense and is smooth in its surface, keeping the advantages of the known winding processes.
This and other objects of the present invention will become apparent from the description hereinafter.